PROJECT SUMMARY Antiviral RNA silencing (AvRS), also referred to as antiviral RNA interference (RNAi), acts as a major antiviral innate mechanism in fungi, plants, and invertebrates. Recent observations suggest that AvRS is also active in undifferentiated mouse cells and appears to be essential for developing mouse babies to fight against lethal viral pathogen. Virus destruction in AvRS is guided by small interfering RNAs derived from viral double-stranded RNAs (dsRNAs), usually the replication intermediates. Therefore, genetic mutations that accumulate in the viral genome during the term of virus replication do not confer resistance to AvRS. Hence, mechanistic study of AvRS holds promise for developing novel strategies for the treatment of viral infection caused human diseases. In mammals, interferon mediated antiviral immunity represents a major innate immunity against viral infection. This antiviral immunity is often triggered upon the detection of invading viral RNAs by three closely related RNA helicases termed RIG-I-like RNA helicases (RLHs). Among these three RLHs, RIG-I and MDA5 detect virus-produced double-stranded RNAs (dsRNAs) in a sequence- independent manner and, thus, are also resistant to genetic changes within the viral genome. Thus, findings from the study of RLH mediated virus detection is also expected to facilitate the development of novel antiviral therapies. Increasing evidence suggests that RIG-I also plays essential role in regulating mammal development. Currently, it remains to be an open question how RIG-I regulates development in mammals Viruses naturally infect and trigger AvRS in Caenorhabditis elegans, making C. elegans an ideal model organism for the study of AvRS. Compared to other model organism, C. elegans has so far the most, in terms of type species, genes identified as key components of AvRS. Because of its short life span and genetic tractability, C. elegans also allows for rapid identification of novel AvRS genes for in-depth study of AvRS. More importantly, accumulating evidence suggests that worm RLHs contribute to both AvRS, by acting as a virus sensor, and worm development. Thus, C. elegans as a model system would serve us well in addressing the question how the virus detection function of RLHs is regulated and how RLHs contribute to the regulation of development. This application seeks to work on (1) genetic and functional characterization of the candidate AvRS genes isolated from a random genetic screen; (2) mechanistic study of worm AvRS initiation; (3) mechanistic study of viral transcript destruction by worm AvRS. Findings from the proposed research are expected to not only improve our understanding of worm AvRS but also facilitate our study on the regulation of RLH function in virus detection and the regulation of development by RIG-I in mammals.